Two Goblin Electromancers will make instant and sorcery spells you cast cost less to cast, and so on.

Goblin Electromancer's effect reduces only generic mana in the spell's total cost.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.